I've been looking for a power feed left minimag body, and would buy the whole marker for the right price. Is there a reason you started at 300?
Reverse classic valves were standard. The minimag cam standard with a "Reverse valve" as well as getting from the factory, guns set up specifically
You are putting the car before the cart and horse. The TW adapters only came out way after the ULE bodies, and i suspect mostly for the Original RT users